DIBUJOS CON ADOBE FLASH

As mentioned in Chapter 4, the hole size is defined by the ratio of the radius of the hole to the radius of the maximum possible hole in a platelet. Therefore the hole size is directly related to the ratio of R/H, R/W and R/L. Thus vaulted structures with various hole sizes should exhibit different compressive strengths and porosities. In order to test this hypothesis, the following experiments were carried out.

Experimental Arrangement

Cylindrical samples (Ø15mm × 30mm) of vaulted structures with four platelets in each unit cell were designed. The aspect ratio of the cell was set to 1: 1: 1 (x: y: z) to ensure that the hole size was equal to the ratio of R/H, R/W and R/L. A range of hole sizes were used in the range of 0% to 80% in steps of 20% to create vaulted structures with different compressive strengths and porosities. The cell size was set to 900µm to ensure at least seven cells were present in each direction of the

sample so that any edge effects were minimised for the compressive strength test compliant with the protocol devised by Ashby [174]. The samples were fabricated using the SLM machine based on the parameters listed in Table 3-3. Finally the compressive strength of the samples was measured using the Instron 4505 based on ASTM-E9 with the porosity being measured gravimetrically.

Results

Figure 5-7 shows that the compressive strength of the regular vaulted structures decreases as hole size increases from 0% to 80%. The compressive strength drops from 184.80MPa to 41.39MPa. The compressive strength decreases from 184.8MPa to 41.3MPa when the hole size increases from 0% to 80%. The decrease of compressive strength was up to 77%.

Figure 5-7 The change of compressive strength of regular VS at various hole sizes.

In Figure 5-8, the graph shows the porosity increase as the hole size increases and this effect is not linear. This trend however slightly changed when the hole size lies in the range of 0% to 20%. In this range, the porosity decreases as hole size increases. 0 50 100 150 200 250 0 10 20 30 40 50 60 70 80 90 C om p ressiv e st ren gt h , 𝜎 , (MP a) Hole size, R/H, (%)

Four platelets in each unit cell, cell size 900µm

Maximum difference between compressive strength143.41 MPa

Figure 5-8 The change of porosity of regular VS at various hole sizes.

Discussion

The hole in the platelet not only causes the decrease of cross section area but also generates a concentration effect further decreasing that the load the plate can withstand. Based on the stress distribution on a platelet, the stress concentration effect is described by a stress concentration factor in equation ( 5-3 ).

𝜎

𝜎 ( 5-3 )

is the stress concentration factor based on the gross stress

𝜎 is the maximum stress at the edge of the hole

𝜎 is the stress on the cross section far from the hole

The stress concentration factor is determined by the geometry of the hole. As discussed in Chapter 4, the hole in a platelet is approximated by a polygon with a specified number of edges. This results in a complex geometry hole rather than an ideal circle. This complex geometry is related to the stress concentration factor which leads to a non-linear change of compressive strength as the hole size increases. 66 68 70 72 74 76 78 80 82 84 86 0 10 20 30 40 50 60 70 80 90 P or osit y, 𝜌r , ( %) Hole size, (%)

Four platelets in each unit cell, cell size 900µm

For the SLM fabrication, the structure is sliced into layers and so the edge of the part consists of a set of laser scan ends as seen in Figure 5-9. While processing these ends there can be an increase in the local energy density and the excess energy can over melt the region or sinter powder particles to the hole so as to close the hole. Therefore no holes can be seen in Figure 5-10, (when the pre-defined hole size was less than 20%), because the excess energy still partly melts the particles so that they are attached to the platelets and this decreases the porosity.

Figure 5-9 Platelets with different hole size sliced by same layer thickness.

Figure 5-10 The fabricated hole sizes when the pre-defined hole size is less than 20%

Laser scanning path Laser scanning ends

Hole size 0% Hole size 5%

1mm Hole size 10% Hole size 15%

When the pre-defined hole size is more than 20% (180µm), the hole emerges as seen in Figure 5-11. The geometry of the hole is not a circle and its size is defined by the maximum diameter of a circle in the hole. It clearly shows that the pre- defined hole size is larger than fabricated hole size (101µm). As the pre-defined hole size increases, although the excess energy still partly melts the particles and attaches them to the platelets, the fabricated hole size still increases. The increase in the fabricated hole size creates more void space and increases the porosity. This is consistent with the experiment result in Figure 5-8.

Figure 5-11 The fabricated hole size when the pre-defined hole size is 20%

Although the increase in the pre-defined hole size leads to an increase in the fabricated hole size and results in an increase in porosity, the sintered powder particles on the edge hole can reduce this increase. Therefore a smaller change of porosity (10.6%) can be seen. The increase in the fabricated hole size reduces the cross section area and introduces a stress concentration effect, resulting in a larger change of compressive strength (77.5%). This difference between the change of porosity and compressive strength indicates that altering the hole size in a VS is an effective way to enlarge the range of available properties.